302 Mr Newcdl, On Luminosity attending Compression. [Apr. 26, 



27. The contrast of this spectrum with that of the electrically 

 excited phosphorescence (§§ 6, 7) is very striking. In one case of 

 the electrical phosphorescence the oxygen bands were seen, under 

 circumstances however which led me to believe that they were 

 really due to compression. The pressure in the tube for the 

 electrodeless discharge was so low that the electrically excited 

 phosphorescence was very faint, but whilst the phosphorescence 

 was still persisting, more gas was let out of the reservoir into the 

 pump and tube. Without any fresh electrical excitation, the phos- 

 phorescence increased in brilliance and the oxygen bands were 

 seen in the spectrum. In no other cases of electrically excited 

 phosphorescence have I seen any but a continuous spectrum. 



It may be that the difference in the spectra in the two cases 

 will suggest further investigation. I am not able to offer any 

 explanation. 



It is to be regretted that the investigation of the pressure- 

 glow is made so difficult by the apparent importance of what may 

 be described as impurities. I am not able to recover with any 

 certainty the mixture of gases capable of exhibiting the pressure- 

 glow, and can only say that oxygen with traces of nitrogen and 

 S0 2 is the mixture I should begin upon, if I wished to recover 

 the conditions that obtained when I made the experiments about 

 two years ago. 



Possible bearing of the phenomena of pressure-glow on astro- 

 nomical matters. 



28. The origin and mechanism of the luminosity of nebulas and 

 the persistence of their general outlines and details are subjects 

 that have occupied the attention of astronomers at various times ; 

 and the idea of necessarily elevated temperature is losing ground. 



The mere fact that a rarined gas can be got into a state such 

 that compression causes it to glow and emit a band spectrum is 

 suggestive. 



If we admit a slow condensation going on in a gravitating 

 mass of gas, whether it be gathered into globular form or 

 irregularly extended like the nebula in Orion or surrounding 

 stars like the Pleiades, then we should be led by the phenomena 

 of the pressure-glow described in this note to conceive of the 

 possible existence of a surface or layer of luminous matter, outside 

 which luminosity would be inappreciable because the pressure was 

 too low, and inside which the pressure was too high for luminosity 

 of the kind considered. The gradual streaming in of rarified gas 

 through the region when the conditions are favourable to the 

 production of pressure-glow would account for the maintenance of 

 the luminosity and the persistence of outline. 



